Zinc oxide particles have been known as a sunscreen ultraviolet blocking agent in cosmetic product applications. However, the applications of the zinc oxide fine particles, which have been used conventionally, are limited from various points of view. Therefore, a new production method which is able to control a particle diameter and particle shape for enhancing the homogeneity thereof has been desired. The following described about various conventional zinc oxides and methods for production of the zinc oxide.
The zinc oxide particles in Patent Document 1 are formed by aggregation of zinc oxide fine particles in a hexagonal plate shape, and therefore proper slippage cannot be achieved. Further, since the particle shape is not sufficiently controlled, physical properties tend to vary, so that it is difficult to obtain a cosmetic with stable quality.
Patent Document 2 describes flaky plate-shaped zinc oxide particles, and uses thereof in cosmetics and industrial applications. However, many of the flaky plate-shaped zinc oxide particles described in Patent Document 2 have a large particle diameter, and the particle shape is not controlled to be fine and uniform. In the production method, a basic zinc salt is generated, and therefore a thermal decomposition step such as that of calcinating is required to obtain zinc oxide.
Patent Document 3 describes hexagonal plate-shaped zinc oxide particles. However, the zinc oxide particles in Patent Document 3 significantly vary in particle diameter and shape, and are aggregated, so that problems such as those described above cannot be sufficiently rectified.
Patent Document 4 describes hexagonal prism- and hexagonal barrel-shaped zinc oxide particles which can be used as a cosmetic. However, in the described production method, hexagonal prism-shaped zinc oxide particles having a particle diameter of less than 0.5 μm cannot be obtained.
From such a point of view, zinc oxide particles which have a primary particle diameter of 0.1 μm or more and can exhibit higher transparency and higher ultraviolet blocking performance as compared to conventional zinc oxide particles are desired. However, such zinc oxide particles have not been developed.
Patent Document 5 discloses a production method in which a mixed liquid of a zinc compound, acetic acid and glycol is held at a temperature of 50 to 200° C. for 0.5 to 5 hours to generate zinc oxide fine particles having an average particle diameter of 200 nm or less. However, this production method is a method of obtaining a precipitate by heating a solution in which zinc oxide is fully dissolved, and zinc oxide particles obtained by this method are not preferable because particles having an aspect ratio of less than 2.5 cannot be obtained. Further, the production method is not suitable from an industrial point of view because expensive glycol is used.
In applications of heat releasing materials in electronic/electrical fields, aluminum oxide, aluminum nitride, boron nitride, zinc oxide and the like are often used as a heat releasing filler. It is generally known that two or more kinds of spherical particles having different particle diameters are filled, and a filler is filled in a resin or the like at a high rate for achieving high heat releasing performance. Meanwhile, it is considered to make use of anisotropy of thermal conduction by compounding particles of different shapes, such as plate-shaped particles and needle-shaped particles, for more effectively enhancing heat conductivity. However, the zinc oxide particles in Patent Document 1 are aggregates, and therefore influences of heat resistance between particles in plate-shaped particles are significant, so that anisotropy of thermal conduction specific to plate-shaped particles cannot be exhibited.
Usually, in production of zinc oxide particles, mostly a calcinating step is essential. In calcinating, however, equipment capable of performing a treatment at a high temperature is required, and there is the problem that the particle diameter and the particle shape which have been adjusted/controlled in the stage of a zinc oxide precursor are changed by calcinating, and so on. Therefore, it is very preferable to obtain zinc oxide particles without performing calcinating.